Study On Separator Modification And Electrochemical Performance Of Lithium-sulfur Battery

The theoretical energy density of lithium sulfur battery?LSB?is up to 2600 Wh kg^-1,which makes it hold great potentials to be the next-generation energy conversion and storage device.Moreover,sulfur is more attractive in cathode materials,because of its abundant resources,cheap and pollution-free.However,the commercial application of LSBs still have many obstacles,such as the most serious“shuttle effect”initiated by the Lithium polysulfides?Li₂S_x,4?x?8?.In view of the problems existing in lithium sulfur batteries,PMC was synthesized by a simple process using sodium citrate as carbon source and sodium phytate as phosphorus source.The P-doping don't significantly change the structure of carbon frame,so the PMC still have a high pore volume.But,most of the micropores on carbon sheets are changed to mesopores.The high pore volume and reasonable pore size distribution have effectively reinforced the physical constraint ability to polysulfides.Simultaneously,the P-doping isn't very high,but it still has caused bonding disorders and vacancies in PC lattices,which have effectively reinforced the chemical constraint ability to polysulfides.In order to improve the electrochemical performances of LSBs better.NMC was synthesized by a same process using sodium citrate as carbon source and melamine as nitrogen source.The carbon shells connected in the original carbon material are broken by the doping of nitrogen,but the NMC has a larger pore volume.Moreover,most of the micropores on carbon sheets are also changed to mesopores.The high pore volume and reasonable pore size distribution have also effectively reinforced the physical constraint ability to polysulfides.Simultaneously,the higher N-doping has caused the formation of functional groups and lattice defects in porous carbon,providing effectively chemisorption ability to polysulfides.Then,the CNT/PMC/CNT and CNT/NMC/CNT triple-layer structures are formed on PP separators by vacuum filtration processes,which are called as P-SCL and N-SCL,respectively.Element-doped PC layers make the barriers have good chemical and physical adsorption capacity for LiPSs.At the same time,the CNT layers distributed on the surface of SCL not only can provide the coating an outstanding mechanical property to accommodate the volume variation of cathode,but also have excellent conductivity to reduce the polarization effect of electrochemical reactions in the cell.The modified cells show good capacity performances,cycle stabilities and rate performances.The self-discharge performance tests show that the P-SCL and N-SCL not only have better abilities to inhibit the self-discharge of batteries,but also can re-activate and utilize the diffused active materials produced in the self-discharge processes.Even with a high sulfur loading,the cell modified by P-SCL or N-SCL still maintains good capacity performance and cycling stability,indicating the P-SCL or N-SCL coated separator has certain practical value.